Certain forms of induced optical emission, including those associated with Raman detection, are very sensitive to light polarization. In particular, observations made with respect to polarization may be indicative of certain material characteristics such as the degree of crystallinity, which, in turn, may enable the researcher to deduce physical properties such as strength or ductility. As a consequence, it is desirable in many investigations of this type to either stimulate the material with light of a predetermined polarization, or view the induced emissions in terms of polarization, or both.
There do exist fiber-optic-based Raman probe configurations adapted for obtaining polarized data, including instruments now provided by the assignee of this invention. In general, these probes are based upon a counter-propagating backscatter arrangement, as shown schematically with FIG. 1, wherein an excitation beam 102 is merged with a collection path 104 through the use of a combining optic 106. This establishes a combined excitation/collection path 108, enabling sample optic 110 to direct excitation and collection with respect to a common point or localized area 112 of a sample 114.
Using this backscatter arrangement as a foundation, turning now to FIG. 2, polarization analyzers 202 and 204 are typically physically inserted into the collection and excitation path, respectively, as shown, enabling the probe to obtain polarized data. A significant amount of information can be gleaned by measuring all four polarization states, namely: (1) the polarization of both analyzers being vertical and parallel; (2) both analyzers horizontal and parallel; (3) vertical excitation with horizontal collection; and (4) horizontal excitation with vertical collection.
According to the current art, the analyzers 202 and 204 are rotated to obtain the different polarization states. This approach presents a number of drawbacks, however. First, such a method makes it difficult to maintain a precise alignment between the excitation and collection beams. In addition, to obtain the different polarization states, rotary encoders must be attached to the analyzers, and such configurations tend to be quite expensive. An additional problem results from intensity variation due to polarization and stabilities in the excitation path. As a further technical difficulty, if the system is fiber-optic-based, the excitation light delivered through the excitation fiber is elliptical rather than randomly polarized. As such, the major axis of this ellipticity tends to rotate as a function of minor stresses on the optical fiber. When a major axis is aligned with the analyzer, more light will be transmitted than when it is misaligned, resulting in intensity variations in the collected data.